Study On Uhmwpe Pipe Formed By In-mold Winding Method And The Product Performance

Ultra-high molecular weight polyethylene(UHMWPE)is a thermoplastic engineering material with a molecular weight of more than 1.5 million.Due to its extremely high molecular weight,it has excellent performance,such as good wear resistance,weather resistance and impact resistance,therefore,UHMWPE pipes are widely used in mining and transportation,dredging engineering,grain industry and other fields.However,the melt viscosity of UHMWPE is extremely high.The traditional polymer processing technology to prepare UHMWPE pipe not only has extremely low production efficiency but also extrusion instability is prone to occur.The pipe has low weld strength and poor hoop mechanical properties.Therefore,it is of great significance to study the forming method and equipment of UHMWPE pipe with high production efficiency and excellent product performance.In this paper,the melt extrusion behavior of UHMWPE in a speed-controlled high-pressure capillary rheometer is reported,which mimics the traditional method for extruding UHMWPE.The origin of flow resistance and extrusion defects in the UHMWPE extrusion process were studied from a perspective of die structure,die surface energy and material properties.The evolution of extrusion pressure and extrudate appearance of UHMWPE was detailed to clarify its mechanism of extrusion instability,which could be explained by dynamic surface renewal.The extremely long UHMWPE chains adsorb firmly on the high-surface-energy die surface,and the absorbed chains form numerous entanglements with bulk chains.The dynamic variation of this interfacial interaction leads to stress oscillation and surface distortion.Flow modifiers and the low-surface-energy die help to reduce the high stress caused by interfacial interaction.At a high extrusion rate,the effect of die length on the extrusion pressure is almost negligible due to wall slip.The pressure drop depends mainly on the compression ratio and deformation rate of the convergent region.When the elongational stress generated by the convergent flow exceeds the interfacial interaction between the UHMWPE nascent particles,gross melt fractures appear.The above mimetic experiments show that UHMWPE exhibits stick-slip extrusion instability at very low extrusion rates.This paper proposes an innovative forming method of melt sheet in-mold winding forming pipe.UHMWPE powder is melted and plasticized by an extruder,compacted and merged into a melt sheet by mold.The melt sheet enters into in-mold winding forming equipment at a certain angle,and then spiral forward with the help of the spiral edge of the rotating mandrel and barrel of the equipment.Finally,the melt sheet is wound around the mandrel to form a pipe.The mechanical structure of the in-mold winding forming equipment for UHMWPE pipe was designed.From the analysis of the movement and force of the melt in the in-mold winding forming equipment,the matching relationship between the extrusion speed of the melt sheet and the rotational speed of the winding forming equipment was derived,which can provide guidance for optimizing the extrusion process of UHMWPE pipe.The in-mold winding forming equipment for UHMWPE pipe has been successfully developed,and the experimental research was carried out with UHMWPE having a molecular weight of 4.5×10~6 g/mol.The section-by-section disassembly experiment proves the correctness of forming method of melt sheet in-mold winding into a UHMWPE pipe.Since spiral edge of the mandrel and the grooved barrel form a cavity whose volume changes periodically,with the assistance of external heating,the sheet is wound into a well-welded pipe.The self-driven mandrel provides circumferential and axial power for conveying melt,eliminating stick-slip extrusion instability of UHMWPE.At an extrusion speed of pipe up to 43m/h,UHMWPE pipes with good apparent quality can still be obtained.Capillary extrusion experiments show that UHMWPE starts to exhibit extrusion instability when extrusion rate exceeds 5 m/h.The results indicate that the in-mold winding forming method greatly improves the production efficiency of UHMWPE.Maintaining the spiral edge of the mandrel and changing the grooved barrel into a smooth barrel,the output of UHMWPE pipe decreased by15%,and at an extruder rotor speed of 30 r/min,the sheet could not be welded into a pipe.When a smooth mandrel is used,the winding forming machine has no conveying capacity.The residence time and azimuth of movement of the melt in the in-mold winding forming equipment for UHMWPE pipe have been carried on to verify the correctness of the theoretical model.The results show that the experiment and theory have good consistency.The UHMWPE pipe produced by in-mold winding forming equipment has been tested and characterized in many ways.During the process of in mold winding forming UHMWPE pipe,the UHMWPE molecular chain was not significantly degraded,the molecular chain was oriented,and the degree of entanglement decreased.During crystallization,the oriented molecular chain is a heterogeneous nucleation point,the molecular chain crystallizes quickly,resulting in the thinner wafer of the sample.The in-mold winding forming method produces UHMWPE pipes with high welding strength and excellent mechanical properties.The hoop and axial mechanical properties are approximately.For the UHMWPE pipe produced at an extruder rotor speed of 20 r/min,the yield strength is about 20 MPa,the tensile strength is about 34 MPa,the elongation at break is about 450%for the pipe.The abrasion resistance of the pipe is comparable to the highly wear-resistant standard which was prepared by molding with 4×10~6g/mol pure UHMWPE.Since the molecular chains orient in the extrusion direction of melt sheet and the melt sheet was wound around the mandrel to form a pipe,the degree of molecular chains orientation in the circumferential direction of the pipe can be greatly improved.Meanwhile,the convergent channel of the in-mold winding forming equipment induces molecular chains to orient around the axial direction of the pipe.Therefore,the hoop strength of the pipe prepared by the in-mold winding forming method in this paper can be enhanced.By adjusting the winding angle and the compression ratio of the convergent flow channel of the pipe die,molecular chain orientation in the hoop and axial direction of the pipe can be effectively regulated,thereby the pipe with required hoop and axial mechanical properties can be obtained.